Marine drives may be generally classified as inboard, outboard, or inboard/outboard. In an inboard drive, the engine and transmission (or drive) are mounted in the hull and a propeller shaft extends through the bottom of the hull. In an outboard drive, the propeller drive and engine are generally configured as a unit attached to and located outside the hull. Inboard/outboard drives, also referred to as stern drives, have an engine mounted in the hull connected to a drive unit mounted outside of the hull, typically on the stern.
Marine drive units can be further classified as pushing-type and tractor-type. Pushing-type drives generally rely upon propellers facing rearward relative to the boat and generating propulsive force that pushes the boat through the water. Tractor-type drives generally rely upon one or more forward, bow-facing propellers that produce propulsive force to pull the boat through the water. Tractor-type drives may also be referred to as pulling-type drives.
FIG. 1 shows a prior art tractor-type drive arrangement 1 on a boat 2. This arrangement known commercially as the Volvo Penta IPS system, includes an engine with a two-part drive (two engines and drives are shown in the figure). The engine and an upper drive module are mounted in the hull and a steerable lower drive module, or pod, with tractor propellers, is mounted below the hull. The Volvo Penta IPS is considered an inboard drive.
A similar tractor-type drive is described in U.S. Pat. No. 7,226,327, assigned to AB Volvo Penta. FIG. 2 is reproduced from the '327 patent and shows a schematic illustration functionally equivalent to the tractor-type drive 1 from FIG. 1. A pair of forward facing propellers 20, 21 rotate in opposite directions to pull a boat through the water. The propellers 20, 21 are carried on concentric shafts rotatably mounted to an underwater housing 10. The underwater housing 10 is steerable about a substantially vertical pivot axis that coincides with a vertical drive shaft 14, which transmits power from an engine output shaft 16 to the propellers 20, 21. Rotating the underwater housing 10 about the steering axis through the vertical drive shaft 14 directs the propeller force to steer the boat and allows the underwater housing 10 to act as a rudder.